In at least some known CT system configurations, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the "imaging plane". The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a "view". A "scan" of the object comprises a set of views made at different gantry angles during one revolution of the x-ray source and detector.
In an axial scan, the projection data is processed to construct an image that corresponds to a two dimensional slice taken through the object. One method for reconstructing an image from a set of projection data is referred to in the art as the filtered backprojection technique. This process converts the attenuation measurements from a scan into integers called "CT numbers" or "Hounsfield units", which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
During scanning, the x-ray beam is known to spread along a z-axis to form a "scan plane". For each image slice, the object to be imaged often only partially intrudes on the scan plane. Specifically, the object is only partially subjected to the x-ray beam, thus causing inconsistencies in the projection data. When reconstructing an image for a particular slice, these inconsistencies generate incorrect CT numbers, streaks, and other artifacts in generated images. As the slice thickness is increased, the likelihood of partial intrusion increases. The image errors created by partial intrusion are often referred to as "partial volume artifacts".
To reduce partial volume artifacts, known CT systems rely upon human operators to either identify the partial volume artifacts, or to take preventative measures to avoid the generation of such artifacts. The preventative measures include scanning an area of the object, stopping the scan, altering x-ray source collimators, and continuing the scan. For example, a 10 mm collimator which provides a slice thickness of 10 mm, may be used when scanning a region with few bony structures. However, when scanning a region with multiple bony structures, a 3 mm collimator which provides a slice thickness of 3 mm, may be used. This method is both time consuming and cumbersome. Furthermore, this method is neither practical nor efficient when scanning adjacent differing regions. The operator must, based on past experience, select slices of sufficiently small thickness to ensure constant attenuation characteristics across the slice, i.e., to ensure that the object does not partially intrude on the scan plane. However, thin slices typically require significantly long scanning times and x-ray tube cooling delays. Conversely, thicker slices are preferred for improving x-ray photon flux. As a result, the operator is forced to weigh these alternatives and make the proper choice.
Therefore, it is desirable to eliminate operator involvement in the determination of the optimal slice thickness. It is also desirable to eliminate, or substantially reduce, partial volume artifacts without significantly reducing CT system efficiency.